1. Field of the Invention
The present invention relates to a color carrier and a developer used for developing an electrostatic image in, for example, electrophotography, electrostatic recording and electrostatic printing; to an image forming method, a process cartridge and an image forming apparatus which use the developer.
2. Description of the Related Art
In one process through electrophotography, an electrostatic latent image is formed on an image bearing member through charging and light exposing, and then is developed with a developer containing toner to form a toner image. The thus-formed toner image is transferred onto and fixed on a recording medium. Also, toner particles which have been not transferred onto a recording medium; i.e., toner particles remaining on the image bearing member are cleaned with a cleaning member such as a blade provided so as to be in contact with the image bearing member surface.
Meanwhile, toner is produced with, for example, the pulverization method. In the pulverization method, a colorant and an optionally used additive are added to a thermoplastic resin serving as a binder resin, and the resultant mixture is melt-kneaded, pulverized and classified. The toner produced with this method, however, has a large particle diameter, making it difficult to form high-quality images.
In view of this, the polymerization method and the emulsion dispersion method are employed for toner production.
In one known process based on the polymerization method, a monomer, a polymerization initiator, a colorant, a charge controlling agent, etc. are added under stirring to an aqueous medium containing a dispersant to form oil droplets, followed by polymerization (the suspension polymerization method). In another known process, particles are produced through emulsion polymerization or suspension polymerization, and the thus-produced particles are aggregated/fused (the association method).
Such a production method can produce toner with a small particle diameter, but the binder resin of the toner mainly contains a polymerized product obtained through radical polymerization. Thus, there cannot be produced toner whose binder resin mainly contains a polyester resin and/or epoxy resin suitably used for color toner, etc.
In view of this, some patent literatures disclose toner production methods based on the emulsion dispersion method in which a mixture of a binder resin, a colorant, etc., is mixed/emulsified in an aqueous medium (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 05-66600 and 08-211655). This production method can produce toner with a small particle diameter and also, use a wide variety of binder resins. But, undesired microparticles are formed, causing emulsification loss.
In view of this, some patent literatures disclose toner production methods in which a polyester resin is emulsified/dispersed and then the formed particles are aggregated/fused (see, for example, JP-A Nos. 10-020552 and 11-007156). This production method can prevent formation of undesired microparticles, reducing emulsification loss.
However, the toner particles produced through the polymerization method or the emulsion dispersion method tend to be spherical attributed to interfacial tension of the oil droplets formed in a dispersion step, which is problematic. This is because the spherical toner particles are difficult to clean with blade cleaning, since they can pass through the gap between the cleaning blade and the photoconductor while rotating.
In view of this, some patent literatures disclose toner production methods in which particles are mechanically treated with stirring at high speed before completion of polymerization, to thereby deform the particles (see, for example, JP-A No. 62-266550).
However, in this production method, the dispersion state becomes unstable and the particles are likely to agglomerate, which is problematic.
In another known production method, particles are aggregated using, as a dispersant, polyvinyl alcohol with a specific saponification degree, to thereby produce associated particles with a particle diameter of 5 μm to 25 μm.
However, the thus-formed associated particles problematically tend to have a large particle diameter.
Also, some patent literatures disclose toner production methods in which a filler is added to an organic solvent together with a toner composition, to thereby form deformed particles (see, for example, JP-A No. 2005-49858).
However, the filler increases the formed toner in viscoelasticity, resulting in elevation of the lower limit of the fixing temperature thereof. Also, when the filler is caused to be present on the toner surface, the viscoelasticity of the toner does not virtually increase. In this case, however, wax is prevented from exuding or binder resin is prevented from melting to outside, leading to degradation of the low-temperature fixing property and hot offset resistance.
Further, some patent literatures disclose charge controlling agents which are an inorganic layered mineral where interlayer ions (e.g., metal cation) are modified with organic ions or other ions; and use of the charge controlling agents in electrophotographic toners (see, for example, JP-A Nos. 2003-515795, 2006-500605, 2006-503313 and 2003-202708).
Meanwhile, regarding a carrier, an appropriate resin material is generally applied onto the carrier surface to form a firm, strong coat layer. This is performed for the purpose of, for example, preventing filming of a toner component on the carrier surface; making the carrier surface uniform; preventing the carrier surface from oxidation; preventing decrease in moisture sensitivity; extending the service life of the developer; preventing the carrier from adhering to the photoconductor surface; preventing the photoconductor from being scratched and/or delaminated by the carrier; controlling the charge polarity; and adjusting the chargeability.
Regarding the coat layer, various production methods are presented; e.g., a specific resin material is used for forming a coat layer (see, for example, JP-A No. 58-108548); various additives are further incorporated into the coat layer (see, for example, JP-A Nos. 54-155048, 57-40267, 58-108549 and 59-166968, Japanese Patent Application Publication (JP-B) Nos. 01-19584 and 03-628, and JP-A Nos. 06-202381 and 2003-345070); an additive is deposited onto the carrier surface (see, for example, JP-A No. 05-273789); conductive particles with a particle diameter greater than the thickness of a coat layer are incorporated thereinto (see, for example, JP-A No. 09-160304); there is used a carrier-coating material mainly containing a benzoguanamine-n-butyl alcohol-formaldehyde copolymer (see, for example, JP-A No. 08-6307); and there is used, as a carrier-coating material, a crosslinked product between a melamine resin and an acrylic resin (see, for example, Japanese Patent (JP-B) No. 2683624).
However, the carrier produced with any of the above proposed methods has insufficient durability and also, is not sufficiently prevented from adhering to the photoconductor surface. Specifically, the carrier poses problems as to its durability in that, for example, the chargeability becomes unstable in accordance with the occurrence of toner spent on the carrier surface; the resistivity decreases with decreasing of the thickness of the coat layer due to ablasion; and the quality of a printed image gradually degrades in accordance with increase of running in number, although an excellent image can be obtained in an initial state. Thus, the carrier must be improved.
Meanwhile, in an attempt to prevent carrier adhesion and to improve image quality, some patent literatures disclose carriers in which the dynamic and static resistivities thereof are controlled (see, for example, JP-A No. 11-352727).
Furthermore, in recent years, image forming apparatuses have been increasingly required to form images of higher quality at a higher speed. Such a high-speed apparatus considerably applies stress to a developer used. Thus, even when the developer contains a carrier which is conventionally considered to have long service life, the service life of the developer is not sufficiently attained. Separately, carbon black is generally used as a resistivity controlling agent for the carrier. In this case, carbon black is thought to be transferred to the formed color image as a result of film delamination and/or exfoliation of the carbon black, resulting in causing color smear. Hitherto, various countermeasures against this assumed problem have been taken and have exhibited a certain preventing effect.
For example, some patent literatures disclose a carrier in which a conductive material (carbon black) is made to be present on the surface of the core material and not to be present in the resin coat layer (see, for example, JP-A No. 07-140723). Also, some patent literatures disclose a carrier in which the coat layer has a concentration gradient of carbon black (i.e., the concentration of carbon black becomes lower toward the surface the coat layer) and has no carbon black on its surface (see, for example, JP-A No. 08-179570). Further, some patent literatures disclose a dual-coat carrier in which core particles are provided with an inner resin coat layer containing conductive carbon black and the inner layer is provided thereon with an outer resin coat layer containing a white conductive material (see, for example, JP-A No. 08-286429). However, these methods cannot respond to the recent increased stress applied to the developer and sufficiently prevent color smear, which is problematic.
Obviously, one of the most effective countermeasures against color smear is to exclude carbon black responsible for it. However, as described above, when carbon black, having low electrical resistivity, is not used, the resistivity of the formed carrier increases.
In general, in use of a developer containing a carrier with high resistivity, the printed image with a large image area has a very low image density at its center portion and a high image density at only the edge portions, in other words, an image excellent in so-called edge effect can be obtained. Owing to the edge effect, characters and thin lines which are high in image sharpness can be formed, but half-tone images significantly poor in reproducibility are inconveniently formed.
As a resistivity adjuster other than carbon black, titanium oxide, zinc oxide, etc. are known. These compounds, however, cannot be comparable to carbon black in terms of reduction in resistivity of the carrier. Thus, the existing problem is not still solved, and there is still room for improvement.